While the merit of optical fibers as a low-loss, compact communications medium is undisputed, there remains the problem of interfacing the fibers to other fibers and to detectors and sources. Any lossiness or bulkiness of the interfacing devices severely undercuts the benefits of the fiber medium. U.S. Pat. No. 4,329,017 (issued May 11, 1982) and co-pending application Ser. No. 325,256, now U.S. Pat. No. 4,479,697 (issued Oct. 30, 1984), describe a variety of loss-loss interface modules utilizing reflective imaging surfaces. The modules, through the use of partially transmissive surfaces, provide a variety of monitoring, splitting, switching, and multiplexing functions. The modules are characterized by the precise tolerances required in high-capacity optical communications systems and yet may be mass-produced at reasonable cost.
While the aforementioned modules are generally concerned with interfacing a relatively small number of fibers, there are certain applications where a large number (say 10-90) of fibers must be interconnected. For example, a star coupler is a device which allows any one of a plurality of input fibers to communicate with all of a (usually corresponding) plurality of output fibers. This is typically effected by communicating the optical signals from the input fibers into an integrating element which operates to mix such signals, and then uniformly splitting the resultant mixed signal into the output fibers. A typical star coupler construction entails removing the cladding from an intermediate region of each of a plurality of fibers to be intercoupled, and fusing the exposed cores together. The fused portion defines the integrating element, the clad portions of the fibers on one side define the input fibers, and the clad portions on the other side define the output fibers. Alternately, the end portions of the input fibers may be etched to reduce the cladding to minimal thickness, and the ends brought together and cemented to a separate integrating element. (The output fibers are handled similarly.) The fiber ends may be cemented in a linear array to the edge of a thin glass slab or in a generally circular bundle to the end of a circular rod.
A typical use for a star coupler is in a local area network where a number of terminals exchange data with one another. However, for such an application, the coupler may also be required to provide monitoring signals, each representative of the signal on a corresponding channel incoming to the coupler. The monitoring signals are communicated to a central monitoring processor which operates to prevent more than one terminal from transmitting at a given time. Provision of such monitoring signals normally requires tapping into each channel, which tends to be inconvenient and expensive in a system with a large number of channels.